The present invention relates to a lens shutter camera having a small triplet type picture-taking lens.
It is strongly demanded that the picture-taking lens of a lens shutter camera is small and inexpensive. A triplet type picture-taking lens at the rear of which a diaphragm is arranged suitably meets this demand. For example, triplet type picture-taking lenses meeting the demand are disclosed in the examples 1 to 4 of Japanese Patent Publication Open to Public Inspection No. 13071/1995, each example of Japanese Patent Publication Open to Public Inspection No. 97111/1992, and each example of Japanese Patent Publication Open to Public Inspection No. 133016/1989.
On the other hand, a camera system to which a new film standard is applied has been developed recently. For example, on page 11 of the December number of 1994 of "Photographic Industry", there is a description of a new system in which films of 24 mm width are used and 3 types of image planes are provided. Compared with the conventional system in which roll films of 35 mm width are used, the image plane size of the new system is small. Accordingly, a diameter of the permissible circle of confusion is small. Therefore, it is necessary to provide a camera of high image formation performance having a picture-taking lens suitable for this new system.
In order to allow a camera having a triplet type picture-taking lens to be compact, it is necessary to reduce the entire length of the lens including the back focal length with respect to a predetermined image plane size. In order to meet the requirement, there are two methods.
(1) One is to reduce the telephoto ratio.
(2) The other is to reduce the focal distance so that the lens can be of a wide angle.
In any cases, it is difficult to obtain a flat image surface because the curvature of the image surface is deteriorated, Which is peculiar to a triplet lens. That is, a sagittal image surface becomes to be greatly "under" at an intermediate field angle, and it becomes to be greatly "over" at the maximum field angle. Therefore, it is difficult to conduct correction. When the telephoto ratio is reduced, coma is greatly deteriorated, and when the focal distance is reduced so as to make the angle to be wide, the Petzval's Sum is increased, so that it becomes difficult to correct the aberration. When the angle of the lens is made to be wide, not only the aberration is deteriorated but also a ratio of illumination of edge of image field is decreased. Accordingly, in the case of a triplet type lens at the rear of which a diaphragm is arranged, it is not practical that the field angle is set to be not less than 80.degree..
In the cases of lenses described in the examples 1 to 4 of Japanese Patent Publication Open to Public Inspection No. 13071/1995, each lens is made of glass, and when the entire length of the lens including the back focal length is L, and also when the length of a diagonal line of the image surface is D, the value of L/D is approximately 0.8 which is relatively low, however, it is not sufficient. Further, the cost is higher than that of lens made of synthetic resin.
In the cases of lenses described in each example of Japanese Patent Publication Open to Public Inspection Nos. 97111/1992 and 133016/1989, the lens is composed of two component lenses made of synthetic resin, which is inexpensive, however, the value of L/D is approximately 0.9 to 1.0, which is large. Even if the minimum F-number is set at a large value in a practical range so that an allowable amount of aberration can be increased, it is difficult to reduce the value of L/D to be not more than 0.8 or 0.7.